1. Field of the Invention
The invention relates to radiation curable powder coating compositions which crosslink to lightfast and weather-stable films.
2. Discussion of the Background
Thermally crosslinkable powder coating materials are known and are widely used in the paint processing industry.
For example, DE-C 27 35 497 describes PU powder coatings having outstanding weathering stability and heat stability. The crosslinkers whose preparation is described in DE-C 27 12 931 are composed of ε-caprolactam-blocked isophorone diisocyanate containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups, their isocyanate groups again being blocked.
The disadvantage of these systems lies in the elimination of blocking agent during the thermal crosslinking reaction. Since, consequently, the blocking agent may be emitted into the environment, it is necessary on ecological grounds and industrial hygiene grounds to take special precautions to clean the outgoing air and/or to recover the blocking agent. Moreover, the reactivity of the crosslinkers is low. Curing temperatures above 170° C. are required.
Both disadvantages—emission of blocking agent and curing at high temperatures—can be removed by using powder coating materials which are cured by radiation instead of thermally. Radiation curable powder coating materials of this kind are known and are described in, for example, U.S. Pat. No. 3,485,732, EP 0407826, EP 0636669, WO 99/14254, U.S. Pat. No. 3,974,303, U.S. Pat. No. 5,639,560 and EP 0934359.
EP 0636669 and WO 99/14254 describe two-component, radiation curable powder coating materials based on an unsaturated polyester and a vinyl ether. The coatings produced from them are unsuitable for outdoor use due to their severe yellowing.
U.S. Pat. No. 3,974,303 describes thermoplastic resins containing from 0.5 to 3.5 polymerizable unsaturated groups per 1000 g molecular weight. The polyurethanes described (see Example 1), however, are not weather-stable and are of poor flexibility due to the absence of polyester groups and the low chain length.
U.S. Pat. No. 5,639,560 describes radiation curable powder compositions comprising special crystalline polyesters, additionally containing methacrylic end groups, as binders. These powder compositions optionally comprise ethylenically unsaturated oligomers, including urethane acrylates, in minor amounts, preferably up to 10 percent by weight. The radiation curable powder coating formulations presented in this patent, however, have significant disadvantages. They can be ground only at −80° C. and, in general, the resultant coatings are either not stable to outdoor weathering or not flexible. A particularly serious disadvantage is that of the very low pendulum hardness (according to König) of below 120 s.
EP 0934359 describes powderous radiation curable mixtures of amorphous and crystalline polyesters containing terminal methacrylate groups. The powder coating materials produced from them do possess a higher glass transition temperature than the crystalline methacrylates of U.S. Pat. No. 5,639,560. However, cooling is still required for their grinding. Moreover, the hardness and adhesion of coatings are no more than acceptable.